(1) Field of the Invention
This invention relates generally to a carbon brush used for miniature motors having permanent magnet fields and the method of making the same, and more particularly to a carbon brush for miniature motors, which is a metal-filled graphite brush having excellent commutating properties and wear resistance and improved environmental resistance, and is formed by purifying graphite powder material so that the ash content of the graphite powder is reduced to less than 0.05 wt. %, adjusting the particle size of the purified graphite powder, mixing the graphite powder with metal powder, and pressure-forming and sintering the mixture.
(2) Description of the Prior Art
Carbon brushes for miniature motors have heretofore been manufactured by adding a binder to graphite powder purified to approximately to 98% or 99.5%, grinding and screening the solidified mixture, blending metallic powder with the ground and screened mixture to impart desired electrical conductivity as necessary, and then pressure-forming and sintering the resulting mixture.
FIG. 10 illustrates the conventional manufacturing process of carbon brushes for miniature motors, using graphite powder having a purity of 98% to 99.5%.
As shown in the figure, a carbon brush is manufactured by adding a binder to the graphite powder purified to a purity of 98% to 99.5%, grinding and screening the solidified graphite-binder mixture, blending the ground and screened mixture with metal powder to impart desired electrical conductivity, and then pressure-forming and sintering the resulting mixture.
To eliminate the use of the binder, a so-called copper-plated graphite brush is known. The copper-plated graphite brush is manufactured by copper-plating particles of graphite powder which is purified to approximately 99%, then pressure-forming and sintering the copper-plated graphite powder without adding a binder.
The conventional metal-filled graphite brush is manufactured by physically blending natural graphite with a binder, and grinding and screening the mixture. With the physical blending process alone, however, 0.5 to 1.0 wt. % of SiO.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, silicates, MnO, MgO and other oxides as impurities are left in the graphite in the form of ashes.
Even the metal-filled graphite brush manufactured by adding a binder to the graphite powder of the aforementioned purity has good environmental resistance because the amount of the remaining binder is reduced at the time of sintering, and metal particles having a small surface area are less subject to attack by corrosive gases and oxidation.
The carbon brush plated with copper and other metal has a porosity of 10% to 30%. This makes the surface area of the thin-film metal large, leading to high susceptibility to oxidation and attack by corrosive gases.
In this way, the impurities remaining in relatively large quantities tend to damage the lubricating film on the commutator, accelerating the wear of the brush and deteriorating commutation properties.
FIG. 11 is an oscillograph waveform for the conventional metal-filled graphite brush. As is evident in the figure, the waveform of the motor current is considerably irregular. As the wear of the carbon brush proceeds, the insulating material contained in the carbon brush appears on the sliding surface between the brush and the commutator, deteriorating commutation performance, resulting in motor failure in extreme cases.